A pixel sensor comprises an array of pixel sensor cells that detects two dimensional signals. Pixel sensors include image sensors, which may convert a visual image to digital data that may be represented by a picture, i.e., an image frame. The pixel sensor cells are unit devices for the conversion of the two dimensional signals, which may be a visual image, into the digital data. A common type of pixel sensors includes image sensors employed in digital cameras and optical imaging devices. Such image sensors include charge-coupled devices (CCDs) or complementary metal oxide semiconductor (CMOS) image sensors.
While complementary metal oxide semiconductor (CMOS) image sensors have been more recently developed compared to the CCDs, CMOS image sensors provide an advantage of lower power consumption, smaller size, and faster data processing than CCDs as well as direct digital output that is not available in CCDs. Also, CMOS image sensors have lower manufacturing cost compared with the CCDs since many standard semiconductor manufacturing processes may be employed to manufacture CMOS image sensors. For these reasons, commercial employment of CMOS image sensors has been steadily increasing in recent years.
Referring to FIG. 1, an exemplary prior art semiconductor structure comprises an image sensor element comprising three pixels that are sensitive to three different wavelength of light. The wavelength-dependent sensitivity of the pixels is effected by placing color filter materials in the optical path in each of the pixels. For example, a first pixel 200A may be sensitive to a first wavelength range corresponding to red light, e.g., around 680 nm. A second pixel 200B may be sensitive to a second wavelength range corresponding to yellow light, e.g., around 575 nm. A third pixel 200C may be sensitive to a third wavelength range corresponding to green light, e.g., 510 nm. A red filter 190A is provided in the optical path of the first pixel 200A to pass light in the first wavelength range and to block light outside the first wavelength range. A yellow filter 190B is provided in the optical path of the second pixel 200B to pass light in the second wavelength range and to block light outside the second wavelength range. A green filter 190C is provided in the optical path of the third pixel 200C to pass light in the third wavelength range and to block light outside the third wavelength range.
Each pixel is formed on a semiconductor substrate 108 employing semiconductor processing methods known in the art. The semiconductor substrate 108 comprises a heavily-doped semiconductor layer 110, a lightly-doped semiconductor layer 112, and shallow trench isolation structures 120. The heavily-doped semiconductor layer 110 and the lightly-doped semiconductor layer 112 have a doping of the same conductivity type, which is herein referred to as a first conductivity type. Each pixel comprises a photosensitive diode, which comprises a charge collection well 132 having a doping of a second conductivity type and a portion of the lightly-doped semiconductor layer 112, which is located directly underneath the charge collection well 132 and is herein referred to as a semiconductor portion 114. The second conductivity type is the opposite of the first conductivity type. Each pixel further comprises a floating drain 140 having a doping of the second conductivity type, a gate electrode assembly 160 controlling flow of charges from the charge collection well 132 to the floating drain 140, and an optical lens 172. A back-end-of-line (BEOL) interconnect structure 170 is provided between the semiconductor substrate 108 and the optical lenses 172 to provide structural support and electrical wiring of the components of each pixel (200A, 200B, or 200C). A dielectric layer 180 is provided between the optical lenses 172 and the various color filters (190A, 190B, 190C).
The materials for the various color filters (190A, 190B, 190C) typically comprise acrylate, methacrylate, epoxy-acrylate, polyimide, or a combination thereof. The various color filters (190A, 190B, 190C) have refractive indices from about 1.2 to about 1.7, and typically have a thickness from about 300 nm to about 3,000 nm. Each wavelength range requires a different filter material. For an image sensor element having three pixels each sensitive to light in three different wavelength ranges, three different color filter materials need to be repetitively applied and patterned. Repeated application and patterning of different color filter materials increase processing complexity, processing time, and processing cost.
In view of the above, there exists a need for a CMOS image sensor structure that provides pixels having different wavelength sensitivity without employing color filters, and methods of manufacturing the same.
Further, there exists a need for a CMOS image sensor pixel structure that provides such pixels and is compatible with high performance semiconductor devices, and methods of manufacturing the same.